The present invention relates to a sound volume control circuit that controls sound volume by digitally changing an output voltage.
A headphone amplifier, or a speaker amplifier, has a function that controls sound volume. For example, a sound signal processor disclosed in Japanese Patent Application Publication number 2008-109560, and the like are known.
FIG. 2 is a block diagram illustrating a configuration of a sound volume control circuit according to a conventional technique. An inverting input of an operational amplifier 1 is connected to a voltage source of an input voltage VIN via a resistor R0, and a non-inverting input of the operational amplifier 1 is connected to a voltage source of a reference voltage VREF. Between an output and the inverting input of the operational amplifier 1, a digital variable resistor circuit 2 that controls sound volume (that is, which discretely changes a resistance value) is connected in a negative feedback manner. The variable resistor circuit 2 includes a plurality of resistors R1 to Rm that have different resistance values to each other, and are connected in parallel. The resistors R1 to Rm are connected to switches SWa1 to Swam, respectively. The variable resistor circuit 2 operates as a 1-dB-step volume device (for example, 0 dB when the switch SWa is on, −1 dB when the switch SWa2 is on, −39 dB when the switch SWam−1 is on, −40 dB when the switch SWam is on, and so on), for example, by turning on any one of the switches SWa1 to Swam connected respectively to the resistors R1 to Rm. The sound volume control circuit in FIG. 2 switches on/off the switches SWa1 to SWam to change a resistance value of the variable resistor circuit 2 (hereinafter, referred to as RVOL) according to a desired sound volume setting, and therefore an output voltage VOUT is changed.
In the digital sound volume control circuit as illustrated in FIG. 2, in a case of outputting a signal, when the resistance value RVOL of the variable resistor circuit 2 is changed, due to this change, fluctuation of the output voltage VOUT occurs and zipper noise appears. The larger the amplitude of the signal outputting, the larger a voltage difference before and after changing the resistance value RVOL of the variable resistor circuit 2 becomes, and the more prominent the zipper noise becomes.
In order to avoid the zipper noise, it is necessary to make a step size of the resistance value RVOL of the variable resistor circuit 2 smaller. Therefore, an additional resistor and switch are needed per different resistance value RVOL, and a die size increases. Accordingly, a trade-off occurs between the suppression of the zipper noise and the increase of the die size. In addition, there may be a case where problems such as deterioration of a ripple removal rate and the like occur, due to the increase of the number of switches.